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Brucite is often associated with calcium carbonate in calcareous scales. In this study, we focus on the effects of brucite dissolution on the carbonate precipitation. Brucite-saturated solutions are elaborated by adding the same amount of brucite powder in various volumes of different natural and artificial seawaters (natural organic materials are also added, with or without Mg2+ ions). The Mg2+ and Ca2+ concentrations in seawater at the beginning and end of the experiments are determined by ICP-AES. The solid powders are ex-situ analyzed using XRD and SEM. It is observed that brucite dissolution leads to pH increase and enrichment of seawaters in magnesium ions. The Mg2+ ions, either pre-existing in the solution or added from brucite dissolution play a key role in CaCO3 polymorph selection. In order to separate the Mg2+ and OH– effects, CaCO3 precipitation is induced by NaOH addition in artificial seawater with or without Mg2+. Under both NaOH and Mg­(OH)2 addition, only aragonite is precipitated from artificial and natural seawaters in our conditions. NaOH addition in Mg-free seawater allows us to predominantly obtain vaterite or calcite polymorphs, while both aragonite and calcite precipitates are observed from Mg-free seawater when NaOH is replaced by Mg­(OH)2. Natural organic materials added in artificial seawater have a significant effect on aragonite morphology.

The effect of montmorillonite and kaolinite, most common clay in marine water, on nucleation and growth of calcium carbonate in standard sea water was studied. Crystallization was induced by the degasification of the dissolved carbonic gas. It was shown by XRD and SEM analysis that CaCO 3 crystallize under its aragonite polymorph some either the clay concentration or type. It was also found that tested clays inhibited significantly the crystallization of calcium carbonate, especially for concentrations higher than 25 mgL -1. From the fine analyses of the formed solid, it was suggested that the tested clays have an indirect effect on nucleation and growth of aragonite by increasing the Mg ions concentration, strong inhibitor of CaCO 3 formation, in the neighbourhood of clay particles where supersaturation is the higher and than crystallization can occur. In addition to its indirect role, kaolinite can interact with aragonite by adsorbing on their faces and blocking growth sites.